Corrosion fatigue crack propagation behavior of AZ31 magnesium alloy friction stir welded joint

In this paper,the corrosion fatigue crack propagation behavior of AZ31 magnesium alloy friction stir welded joint was studied. The results showed that the corrosion fatigue crack growth threshold and strength in all areas of AZ31 magnesium alloy joints in 1%（ mass fraction）Na Cl solution were lower than that in the air environment,but the crack growth rate was higher. In the air environment,the base metal areahad the minimum threshold and the crack was the easiest to expand in the initial crack extension stage. In the region of the stability of crack extension,the crack propagation rate was the fastest in the welding nuclear zone,da/dN = 7.80×10^-6(ΔK)^2.78,while the crack propagation rate was the slowest in advancing side of thermal-mechanically affected zone,da/dN = 1.94×10^-5(ΔK)^1.73. In the corrosion fatigue environment,the threshold value of the base metal zone was the minimum,and the propagation behavior was the most prone to occur. In the stage of stable extension,thermal-mechanically affected zone in retreating side expanded the fastest,da/dN =3.12×10^-4(ΔK)^1.71,while welding nuclear zone expanded the slowest,da/dN = 2.78×10^-4(ΔK)^1.50. The mechanism of corrosion fatigue crack propagation in various areas of AZ31 magnesium alloy friction stir welded joints was mainly the mechanism of anodic dissolution and hydrogen embrittlement at the crack tip. The crack propagation path was tortuous and complex,“closed-loop”and furcation morphology of crack were found in central crack. The tail of the crack was fine,and the crack“jumps”occurred. Schmid factors near the crack tip unevenly distributed,the microstructure near the crack tip integrally presented a soft orientation,while cracks always propagated along soft orientation tissues that were prone to slip.